System for Measuring Periodic Structures

ABSTRACT

A periodic structure is illuminated by polychromatic electromagnetic radiation. Radiation from the structure is collected and divided into two rays having different polarization states. The two rays are detected from which one or more parameters of the periodic structure may be derived. In another embodiment, when the periodic structure is illuminated by a polychromatic electromagnetic radiation, the collected radiation from the structure is passed through a polarization element having a polarization plane. The element and the polychromatic beam are controlled so that the polarization plane of the element are at two or more different orientations with respect to the plane of incidence of the polychromatic beam. Radiation that has passed through the element is detected when the plane of polarization is at the two or more positions so that one or more parameters of the periodic structure may be derived from the detected signals. At least one of the orientations of the plane of polarization is substantially stationary when the detection takes place. To have as small a footprint as possible, one employs an optical device that includes a first element directing a polychromatic beam of electromagnetic radiation to the structure and a second optical element collecting radiation from the structure where the two elements form an integral unit or are attached together to form an integrated unit. To reduce the footprint, the measurement instrument and the wafer are both moved. In one embodiment, both the apparatus and the wafer undergo translational motion transverse to each other. In a different arrangement, one of the two motions is translational and the other is rotational. Any one of the above-described embodiments may be included in an integrated processing and detection apparatus which also includes a processing system processing the sample, where the processing system is responsive to the output of any one of the above embodiments for adjusting a processing parameter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/016,148,filed Dec. 17, 2004; which is a continuation of application Ser. No.10/748,975, filed Dec. 29, 2003, now abandoned; which is a continuationof application Ser. No. 09/742,029, filed Dec. 20, 2000, now U.S. Pat.No. 6,721,052. These applications are incorporated in their entirety byreference as if fully set forth herein.

BACKGROUND OF THE INVENTION

This invention relates in general to optical techniques for measuringperiodic structures, and in particular to an improved spectroscopicdiffraction-based metrology system for measuring periodic structures,such as grating-type targets on semiconductor wafers.

In conventional techniques, optical microscopes have been used formeasuring the critical dimension (“CD”) for semiconductor lithographicprocesses. However, as the CD becomes smaller and smaller, it cannot beresolved at any practical available optical wavelengths in opticalmicroscopy. Scanning electron microscope technology has extremely highresolution. However, this technology inherently requires large capitalexpenditures and heavy accessory equipment such as vacuum equipment,which makes it impractical for integration with lithographic processes.Two-theta scatterometers face similar practical challenges forintegration, as they require mechanical scanning over a wide range ofangles. For this reason, they are slow and difficult to integrate withprocess equipment.

Ellipsometric techniques have also been used for CD measurements. InU.S. Pat. No. 5,739,909, for example, Blayo et al. describe a method ofspectroscopic ellipsometry adapted to measure the width of features inperiodic structures. While ellipsometric methods may be useful for someapplications, such methods have not been able to measure reflectancesfrom the periodic structures.

None of the above-described techniques is entirely satisfactory. It istherefore desirable to provide an improved technique for measuringperiodic structures where the above-described difficulties arealleviated.

SUMMARY OF THE INVENTION

In many of the diffraction-based metrology or ellipsometric techniquesfor measuring the critical dimension, theoretical models employinglibraries or non-linear regression are employed to find the criticaldimension or other parameters of the periodic structure. Where theperiodic structure measured is topographically complex, it may benecessary to measure more than one radiation parameter to get adequateinformation for the modeling process. Where the periodic structure issimple, however, the measurement of a single radiation parameter may beadequate. The use of fewer radiation parameters in the modeling processreduces the calculation time required so that the critical dimension andother parameters of the structure can be found quickly. The embodimentsof this invention are simple in construction and flexible and may beused for measuring one or more radiation parameters from the radiationdiffracted by the periodic structure.

In one embodiment of the invention, when the structure is illuminated bypolychromatic electromagnetic radiation, radiation from the structure iscollected and divided into two rays having different polarizationstates. The two rays are detected, from which one or more parameters ofthe periodic structure may be derived. In another embodiment, when theperiodic structure is illuminated by polychromatic electromagneticradiation, the collected radiation from the structure is passed througha polarization element having a polarization plane. The element and thepolychromatic beam are controlled so that the polarization planes of theelement are at two or more different orientations with respect to theplane of incidence of the polychromatic beam. Radiation that has passedthrough the element is detected when the planes of polarization are attwo or more positions so that one or more parameters of the periodicstructure may be derived from the detected signals. At least one of theorientations of the plane of polarization is substantially stationarywhen the detection takes place.

When a device for measuring periodic structure parameters is employed ina production environment, such as when it is integrated with waferprocessing equipment, it is desirable for the device to have as small afootprint as possible. One embodiment of the invention adapted for suchenvironments employs an optical device that includes a first elementdirecting a polychromatic beam of electromagnetic radiation to thestructure and a second optical element collecting radiation from thestructure where the two elements form an integral unit or are attachedtogether to form an integrated unit.

One way to reduce the footprint of the apparatus for measuring theperiodic structure in a wafer processing environment is to move theapparatus relative to the wafer without moving the wafer itself.However, since the apparatus includes a number of components, it alsohas a significant size and footprint compared to that of the wafer.Furthermore, it may be cumbersome to control the sizable apparatus sothat it can move in a two dimensional plane without moving the wafer.Thus, it is envisioned that both the apparatus for measuring theperiodic structure and the sample (e.g. wafer) are caused to move. Inone embodiment, translational motion of the apparatus and the wafer iscaused where the two motions are transverse to each other. In adifferent arrangement, one of the two motions is translational and theother is rotational. This facilitates the handling of the motion of theapparatus while at the same time reduces the overall footprint.

Any one of the above-described embodiments may be included in anintegrated processing and detection apparatus which also includes aprocessing system processing the sample, where the processing system isresponsive to the output of any one of the above embodiments foradjusting a processing parameter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic view of an apparatus for measuring one or moreparameters of the periodic structure to illustrate one embodiment of theinvention.

FIG. 1B is a schematic diagram of the plane of incidence and the planesof polarization of the polarizer and analyzer of FIG. 1A to illustratethe operation of the apparatus of FIG. 1A.

FIG. 1C is a schematic diagram of another apparatus for measuring one ormore parameters of the periodic structure to illustrate an alternativeembodiment of the invention particularly suitable in an integratedprocessing and detection tool to illustrate another embodiment of theinvention.

FIG. 2 is a schematic diagram of yet another measurement device formeasuring one or more parameters of the periodic structure to illustrateyet another embodiment of the invention.

FIG. 3 is a schematic diagram of illumination and collecting opticsemployed in the different embodiments of this invention to illustrate anasymmetric numerical aperture for illumination and collection.

FIG. 4A is the perspective view of a semiconductor wafer and anapparatus for measuring periodic structures on the wafer and instrumentsfor moving the apparatus and the wafer to illustrate yet anotherembodiment of the invention.

FIG. 4B is a perspective view similar to that of FIG. 4A except that theapparatus of FIG. 4B is self-contained in the optical head and includesa polychromatic electromagnetic radiation source.

FIG. 4C is a schematic view of a semiconductor wafer and of opticsarranged so that the optical path length remains substantially constantirrespective of the positions of the moving optics head relative to thewafer and illustrates yet another embodiment of the invention.

FIG. 5 is a schematic view of a system for moving the optics head alonga straight line and for rotating the wafer.

FIG. 6 is a schematic diagram of an apparatus illustrating in moredetail the components in the optics head in FIGS. 4A, 4C.

FIG. 7 is a schematic view of an apparatus for measuring periodicstructures illustrating in more detail an alternative construction ofthe optics head in FIGS. 4A, 4C.

FIG. 8 is a schematic view of the different components in the opticshead to illustrate in more detail the optics head of FIGS. 4B, 5.

FIG. 9 is a flowchart illustrating the operation of the apparatuses ofFIGS. 1A, 1C and 2.

FIG. 10 is a schematic block diagram illustrating a wafer processingapparatus including a track/stepper and an etcher and a spectroscopicmeasurement device where information from a periodic structure and/orassociated structures from the device as used to control themanufacturing process and the track, stepper and/or etcher to illustratethe invention.

FIG. 11 is a schematic block diagram and flowchart illustrating in moredetail the track/stepper of FIG. 7.

For simplicity in description, identical components are labeled by thesame numerals in this application.

DETAILED DESCRIPTION OF THE EMBODIMENTS

As shown in FIG. 1, a broadband source of electromagnetic radiation 20provides a polychromatic beam with wavelength components preferably from180 to 1000 nm. In one embodiment, source 20 may be a xenon lamp. Thebeam 22 from source 20 is shaped by beam shaping optics 24. A portion ofthe beam 22 is diverted by a beam divider 26 to spectrometer 28 formonitoring intensity variations in beam 22 in order to normalize theresults of detection. Beam 22 is polarized by a polarizer 30 and thepolarized beam is directed to sample 32 to illuminate a periodicstructure thereon. Preferably the plane of incidence 34 (see FIG. 1B) ofbeam 22 is perpendicular to the grating lines in the periodic structureof sample 32. Radiation originating from beam 22 that has been modifiedby sample 32, such as by reflection or transmission, is collected bycollection optics 42 as a collected beam 40 and passed through ananalyzer 44 which splits the collected radiation into two rays: anextraordinary ray or e-ray 46 and an ordinary ray or o-ray 48. Rays 46and 48 are detected respectively by two spectrometers 52, 54 withoutputs delivered to computer 60 along with the output of spectrometer28 for determining one or more parameters of a periodic structure onsample 32.

Depending on the orientation of the polarizer 30 and analyzer 44, thee-ray and 0-ray contain information concerning two different radiationparameters. This is illustrated more clearly in reference to FIG. 1B. Asshown in FIG. 1B, R_(s) is the reflectance of the S component of beam 40in the direction normal to the plane of incidence and R_(p) is thereflectance of the P component of beam 40 within the plane of incidence34 of beam 22. Assuming that polarizer 30 is oriented at an angle to theplane of incidence 34, where the angle is other than 0°, 90°, 180° or270°, then beam 40 will contain both non-zero P components and Scomponents. When a plane of polarization is said to be at an angle tothe plane of incidence in this application, this angle can be of anyvalue, including 0 and 180 degrees, unless stated otherwise. Forexample, beam 33 may have the input polarization along the plane 62 atan angle between 0 and 90 degrees to plane 34 in FIG. 1B. In such event,depending on the orientation of the plane of polarization of analyzer44, different radiation parameters are measured. For example, if theplane of polarization of analyzer 44 is aligned with the plane ofincidence 34 or perpendicular to the plane of incidence, then thecomponents R_(p), R_(s) are measured, and the o-ray and e-ray wouldyield these two intensities. On the other hand, if the plane ofpolarization of the analyzer 44 is aligned with the input polarizationplane 62 of beam 33, or is arranged to be perpendicular to plane 62,then one of the two rays 46, 48 would contain information concerning asignal X and the other ray would contain information concerning a signalY (a vector perpendicular to signal X), where X and Y are defined asfollows:X=|r _(s) −r _(p)|²Y=|r _(s) +r _(p)|²

where r_(s) and r_(p) denote the complex amplitude reflectioncoefficients for S and P polarizations, respectively, while R_(s) andR_(p) are the intensity reflection coefficients.

If the plane of polarization of polarizer 30 is in the plane ofincidence 34 or perpendicular to it, it is not possible to measure bothR_(p) and R_(s) simultaneously or measure X and Y simultaneously. Thisis true when the grating is perpendicular or parallel to the plane ofincidence 34. In other words, where the grating is not perpendicular orparallel to the plane of incidence 34, it is possible to measure bothR_(p) and R_(s) simultaneously or measure X and Y simultaneouslyirrespective of the plane of polarization of beam 33. It is alsopossible to remove polarizer 30 altogether so that the beam 22illuminating sample 32 is unpolarized. In such event, in order tomeasure R_(s) and R_(p), the plane of polarization of analyzer 44 ispreferably aligned with the plane of incidence 34 or perpendicular toit. Even where the plane of polarization of analyzer 44 is not parallelto the plane of incidence 34 or perpendicular to it, it is possible tomeasure quantities related to R_(s) and R_(p), which would be useful formeasuring the structure.

Preferably the plane of polarization of the analyzer 44 is at an angleof 0, 45, 90, 135, 180, 225, 270 or 315 degrees from the plane ofincidence 34, since this will optimize the amplitudes of the quantitiesmeasured. Where the plane of polarization of the analyzer 44 is at anangle of 0, 90, 180 or 270 degrees to plane 34, R_(p), R_(s) may bemeasured. Where the plane of polarization of the analyzer 44 is at anangle of 45, 135, 225 or 315 degrees to plane 34, X, Y may be measured.Where the polarizer 30 is oriented at one of the angles of 0, 45, 90,135, 180, 225, 270 or 315 degrees from the plane of incidence 34, againthe amplitudes of the quantities measured will be optimized.

After R_(p), R_(s), X, Y have been measured, it is then possible toderive other radiation parameters therefrom in computer 60, so that amore complete list of quantities that can be obtained from themeasurement are the following:R_(s), R_(p), R_(s)−R_(p), (R_(s)−R_(p))/(R_(s)+R_(p))X=|r _(s) −r _(p)|² , Y=|r _(s) +r _(p)|²X/Y,X−Y≡−4|r _(s) ||r _(p)| cos Δ(X−Y)/(X+Y)≡sin 2ψ cos Δcos Δ, ${\tan\quad\psi} = {\frac{r_{p}}{r_{s}}}$where r_(s) and r_(p) denote the complex amplitude reflectioncoefficients for S and P polarizations respectively, while R_(s) andR_(p) are the intensity reflection coefficients: R_(s)=|r_(s)|²,R_(p)=|r_(p)|². Δ denotes the phase difference between reflected s- andp-polarized fields. The quantities tan Ψ and cos Δ are ellipsometricparameters known to those in the art.

The radiation parameter (Rs−Rp)/(Rs+Rp) has better sensitivity than Rsor Rp alone, since the reflectance difference (Rs−Rp) would tend tocancel out unwanted reflectance from isotropic film structuressurrounding, over or underneath the grating targets when theillumination beam is at normal or near normal incidence. The ratio alsoreduces effects of intensity variations of the illumination beam on themeasurement. By canceling out or reducing unwanted reflectance fromisotropic film structures surrounding, over or underneath the gratingtargets, this feature renders the detection system more robust againststray reflectance around the grating target, and reduces the influenceof thickness variations of films over or under the grating. Since thearea available for the grating target is limited, where the opticalsystem is not exactly aligned properly, some of the radiation from theillumination beam may be reflected from an area outside of the target,so that by using the computer 60 to calculate the reflectancedifference, the measurement is much more robust despite opticalmisalignment errors, and reduces the influence of thickness variationsof films over or under the grating. These variations are caused byprocess variations that introduce error in CD measurement. By measuringdifference in reflectance, these unwanted effects tend to cancel toprovide a more accurate CD measurement. As noted above, the output ofspectrometer 28 may be used to normalize the output of spectrometers 52,54 in order to eliminate the effects of intensity variation from source20.

Where it is desirable to employ a compact system for detecting periodicstructures, such as where the device is to be integrated with processingequipment, the configuration shown in FIG. 1C may be particularlyuseful. As shown in FIG. 1C, the incident beam 22 is passed through anelement 30′ which can be a polarizer, focused by optics 24′ to sample32. The zeroth order diffraction from the periodic structure on sample32 is collected by focusing optics 24′ and passed through an analyzer44′. The functions of the polarizer 30′ and analyzer 44′ are similar tothose of polarizer 30 and analyzer 44 of FIG. 1A described above. For acompact design, polarizer 30′ and analyzer 44′ may be a single integralunit or are separate elements attached together to form an integratedunit 62 so that the configuration of FIG. 1C is particularly compact.This allows the apparatus of FIG. 1C to have a particularly smallfootprint.

FIG. 2 is a schematic view of an apparatus 100 for measuring a periodicstructure to illustrate another embodiment of the invention. Forsimplicity, the computer that processes the output of the twospectrometers 28, 54 has been omitted to simplify the drawing. System100 differs from system 10 of FIG. 1A in that in system 100, either oneor both of the polarizer 30 and analyzer 44 may be rotated whereas thepolarizer and analyzer in system 10 of FIG. 1A are stationary and do notmove. Furthermore, system 100 includes a single spectrometer fordetecting the collected radiation instead of two spectrometers as inFIG. 1A. However, by rotating the polarizer 30 or analyzer 44, two ormore radiation parameters may be detected. The operation of system 100is different from an ellipsometer in that, for at least one measurement,the polarizer 30 and analyzer 44 are stationary and do not move. Thispermits system 100 to measure quantities such as R_(s), R_(p) whereas itwould be extremely difficult for an ellipsometer to measure R_(s), R_(p)accurately.

System 100 operates under principles similar to that of FIG. 1A. Thus,if the plane of polarization of polarizer 30 is at an angle differentfrom 0, 90, 180 and 270 degrees to the plane of incidence of beam 22,the plane of polarization of analyzer 44 may be oriented in the samemanner as that described above in reference to FIG. 1B to measure atleast one of the parameter values R_(s), R_(p), X, Y. Since only onespectrometer is used instead of two as in FIG. 1A, the quantities aremeasured sequentially rather than simultaneously as in the embodiment ofFIG. 1A. Thus, in reference to FIG. 1B, if the plane of polarization ofanalyzer 44 is aligned with the plane of incidence 34 of beam 22, thenthe spectrometer would detect the radiation parameter R_(p). Thenanalyzer 44 is rotated so that its plane of polarization issubstantially normal to plane 34; at this stationary position ofanalyzer 44, the spectrometer 52 measures the radiation parameter R_(s).The same can be said for the measuring of X, Y, where analyzer 44 isrotated so that its plane of polarization is substantially parallel andnormal to plane 62, where the measurements are conducted sequentiallyrather than simultaneously. In other words, analyzer 44 may besequentially oriented so that its plane of polarization is substantiallyat some of the angles of about 0, 45, 90, 135, 180, 225, 270 or 315degrees to the plane of incidence 34, in order to measure one or more ofthe parameter values R_(s), R_(p), X, Y.

FIG. 3 is a schematic view of illumination optics 24″ and collectionoptics 42″ to illustrate another aspect of the invention. CDmeasurements require a dedicated grating-type target that is printed onthe scribe line between dies on a semiconductor wafer. The areaavailable for such target is typically limited so that the target sizeis typically kept to a minimum, such as around 25 microns square. Even asmall percentage of light reflected from areas outside of the gratingtarget can affect the measurement accuracy. For this reason, arelatively large focusing numerical aperture is employed to keep theenergy of the illumination beam focused on the small target area. Thus,as shown in FIG. 3, the illumination numerical aperture 72 is relativelylarge. A large numerical aperture in the collection optics increases thecomputing time for modeling since the spectrum at more incident angleswould need to be calculated. For this reason, it may be desirable toemploy a small collection numerical aperture, such as aperture 74 shownin FIG. 3, for the collection optics. While FIG. 3 illustrates thecollection optics in transmission mode, the same can be applied toreflection mode, such as those in FIGS. 1A, 1C and 2.

In another embodiment of the invention, both the apparatus and the waferare moved relative to each other. Where the apparatus is moved in atranslational motion along a straight line, such motion is easy tocontrol despite the size and the number of components in the apparatus.Where the wafer is moved in the direction perpendicular to the motion ofthe apparatus, the footprint of the entire system can be reduced bymaking the dimension of the apparatus elongated along the direction ofmotion of the wafer, in order to minimize the dimension of the apparatusalong the direction of its motion, in order to minimize footprint. Thisis illustrated in FIG. 4A. As shown in FIG. 4A, an optics head 102 ismounted onto a translation track 104 and a motor (not shown) causes head102 to move along the track 104 along direction 106. Track 104 isattached onto supporting base 112, having tracks 115. The semiconductorwafer 32′ is placed onto a chuck 114 which is caused by another motor(not shown) to move along track 115 along direction 116. Wheredirections 106, 116 are transverse to each other, such as perpendicularto each other, the movement of the optics head 102 and wafer 32′ asdescribed above would enable the optics head 102 to inspect the entiresurface area of sample or wafer 32′. As will be evident from FIG. 4A,the optics head 102 is elongated in shape, with the elongated dimensionsubstantially along direction 116, the direction of motion of wafer 32′.This reduces the dimension of optics head 102 in the direction of motion106 of the head, thereby reducing the footprint required. Thus, as shownin FIG. 4A, the footprint of the entire assembly shown in FIG. 4A hasthe dimension along direction 106 substantially equal to the sum of thedimension of wafer 32′ and optics head 102. The footprint of theassembly along direction 116 is substantially twice the dimension ofwafer 32′ along such direction.

FIG. 4B illustrates in perspective view an assembly 120 that is similarto assembly 110 of FIG. 4A, except that assembly 120, the optics headcontains a radiation source and all collection optics and sensorcomponents, so that it has no provision for an optical fiber leading tothe optics head. In FIG. 4A, on the other hand, the optics head 102 doesnot include a source of radiation, so that an optical channel, such asan optical fiber 118, is employed to supply radiation to optics head102. Track 104 has a passage therein to provide a conduit for theoptical channel 118 to the optics head. In FIG. 4B, the optics head 102′contains a radiation source, so that track 104′ does not need to providethe passage for an optical channel.

FIG. 4C is a schematic diagram of a semiconductor wafer and of opticsarranged so that the optical path lengths to all positions of the opticshead remain substantially constant irrespective of the position of theoptics head relative to the wafer to illustrate yet another embodimentof the invention. In FIG. 4C, radiation used for inspection is suppliedalong an optical channel 118. When the optics head is at position 102(1)or D, radiation along channel 118 would reach the optics head 102 byreflection by means of folding mirror 120 (at position 120(1) or C) tothe optics head. In other words, the optical path length would includethe paths BC and CD. In the embodiment shown in FIG. 4C, the wafer ismoved along direction 116 or one parallel to the x axis, and the opticshead is moved along direction 122 substantially at 45 degrees to the xaxis. The folding mirror is moved along the y axis at a speed such thatthe mirror and the optics head are maintained to be along a lineparallel to the x axis or direction 116. Thus, after the optics head andthe wafer are both moved so that the optics head is now at position102(2) or A, and the mirror at position 120(2) or B, the optics headwill receive radiation from channel 118 along a different optical pathAB than it received when it was at position 102(1). Since the line AD isat 45 degrees to direction 116, the optical path length AB is equal tothe sum of the path lengths BC+DC. If the optical beam along channel 118is not collimated, any optical path difference depending on thepositions of the optics head may cause complications in the measurementprocess. At a minimum, this may call for a calibration process which maybe cumbersome. For this reason, in the embodiment of FIG. 4C, the opticsfor directing radiation from channel 118 to different positions of theoptics head is designed so that the total optical path length remainssubstantially the same irrespective of the position of the optics head.

In yet another embodiment, to further reduce the footprint of thesystem, the wafer is rotated while the apparatus for measuring theperiodic structure is moved along substantially a straight line, so thatthe illumination beam from the apparatus scans a spiral path on thewafer. This is shown in FIG. 5. Thus, the optics head 102′ is mountedonto a translation track 104′ as in FIG. 4B and the optics head is movedalong the track along direction 106 by a motor (not shown) as before.The wafer (not shown), however, is situated on top of a support 124which is rotated along direction 126 by a motor (not shown). In thismanner, the optics head 102′ would have access to any location on thewafer for taking measurement.

FIG. 6 is a block diagram of an optical arrangement 140 illustrating apractical implementation for the optics head 102 of FIGS. 4A, 4C. Asshown in FIG. 6, the optics head 102 a contains an optical arrangementsimilar to that in FIG. 1A, where the analyzer 44 splits the collectedbeam 40 into an o-ray directed to spectrometer 54 and an e-ray directedto spectrometer 52 through a prism 141, which compensates for thedispersion caused by the analyzer 44 for the deviated beam. The lightfrom light source is conveyed through an optical fiber 20′ and reflectedby a collimating mirror 142 to polarizer 30 and apodizer 144 in acollimated light piston to optics head 102 a. In optics head 102 a, thelight is reflected by mirror 146 to the beam shaping optics 24, 42comprising two focus/collimating mirrors to sample 32. The samefocus/collimating mirrors 24/42 collect light reflected by sample 32 toanalyzer 44. As will be noted from FIG. 6, collimating or focusingmirrors are used for directing and focusing radiation from the lightsource cross the sample 32 and for collecting light reflected by thesample towards the analyzer. By using mirrors for such purposes,chromatic aberration is reduced, especially where the detection employsradiation over a wide range of wavelengths, such as 150 nanometers to1,000 nanometers.

FIG. 7 is a schematic view of another alternative construction 102 b ofthe optics head in FIGS. 4A, 4C. As shown on FIG. 7, optics head 102 bincludes an optical fiber 20′ conveying radiation from a radiationsource (not shown) through polarizer 30 and apodizer 144 and focused bymirror 152 to sample 32. Sample 32 is moved by a stage 148 along onedirection (x) or along two transverse (x/y) directions. Radiationreflected by sample 32 is collected by collection mirror 154 and focusedto an analyzer 44. The radiation that passes the analyzer is conveyed toa spectrometer (not shown) by fiber 156. Analyzer 44 is rotated todesirable positions as described above in reference to FIG. 2.

FIG. 8 is a schematic view of the different components in the opticshead 102′ to illustrate in more detail the optics head of FIGS. 4B, 5.As shown in FIG. 8, optics head 102′ includes radiation source 20 whichsupplies a beam of radiation that passes polarizer 30 and is reflectedby a beam splitter 162 and focused by a broadband objective 164 to wafer32. The reflected radiation from the wafer is collected by objective164, passed through beam splitter 162 and reflected by mirror 166 toanalyzer 44 to spectrometer 54. Preferably the analyzer 44 is rotated sothat its plane of polarization and the plane of polarization of thepolarizer 30 are as described above in reference to FIG. 2.

The process for deriving one or more parameters of the periodicstructure using a computer such as computer 60 will now be described inreference to FIG. 9 which is a flowchart illustrating the operation ofapparatuses of FIGS. 1A, 1B and 2. First, one obtains the possible rangeof values of the one or more parameters of the periodic structure, suchas critical dimension (CD), height (H), sidewall angle (SWA) and otherparameters (block 202). A theoretical model is then constructed (block204) and a multi-dimensional library is constructed (block 206).

One of the apparatuses of FIGS. 1A, 1C and 2 is then utilized to obtaina measurement of the zeroth order diffraction of the periodic structurein the manner described above (block 208), where such diffraction isdetected by a single or dual channel spectrometer as described above(block 210). As described above, the quantities R_(s), R_(p) and X, Ymay be measured directly using the apparatuses of FIGS. 1A, 1C and 2.From these four quantities, other radiation parameters may be derived inthe manner described above (block 212). The multi-dimensional library206 constructed is then utilized and comparison is made between spectrain the library and the measured data from block 212. The parameters ofthe model are varied (block 206) in order to minimize the error betweenthe spectra corresponding to the predicted values of the parameters ofthe periodic structure according to the model, and the measured spectrafrom block 212 in order to arrive at the actual values of the parametersof the periodic structure, such as CD, height and sidewall angle(diamond 214).

Alternatively, instead of using a multi-dimensional library, anon-linear optimization (e.g. non-linear regression) algorithm may beemployed instead. For this purpose, the initial or seed values of theparameters of the periodic structure are obtained (block 222). Anoptimization model is constructed (block 224) and a non-linearregression algorithm (block 226) is employed with feedback (diamond 214)to again minimize the error between the spectra of radiation parameterscorresponding to the predicted values of the parameters of the periodicstructure and the measured spectra from block 212 in order to arrive atthe actual values of parameters such as CD, height and sidewall angle(block 216).

FIG. 10 is a block diagram of an integrated diffraction-based metrologysystem, a photolithographic track/stepper and an etcher to illustrateanother aspect of the invention. A layer of material such as photoresistis formed on the surface of a semiconductor wafer by means oftrack/stepper 350, where the photoresist forms a grating structure onthe wafer. One or more of the CD, H, SWA and/or other parameters of thegrating structure are then measured using systems 10, 80, 100 of FIGS.1A, 1C and 2, and one or more of the above-described techniques may beemployed if desired to find the value(s) of the one or more parametersof the photoresist pattern. Such value(s) from the computer 60 are thenfed back to the track/stepper 350, where such information may be used toalter the lithographic process in track/stepper 350 to correct anyerrors. In semiconductor processing, after a layer of photoresist hasbeen formed on the wafer, an etching process may be performed, such asby means of etcher 360. The layer of photoresist is then removed in amanner known in the art and the resulting grating structure made ofsemiconductor material on the wafer may again be measured if desiredusing system 10, 80 or 100. The value(s) measured using any one or moreof the above-described techniques may be supplied to the etcher foraltering any one of the etching parameters in order to correct anyerrors that have been found using system 10, 80 or 100. Of course theresults obtained by one or more of the above described techniques insystem 10, 80 or 100 may be used in both the track/stepper and theetcher, or in either the track/stepper or the etcher but not both. Thetrack/stepper 350 and/or etcher 360 may form an integrated single toolwith the system 10, 80 or 100 for finding the one or more parameters ofa periodic structure, or may be separate instruments from it.

FIG. 11 is a schematic view of the track/stepper 350 and an associatedflowchart illustrating a process for semiconductor wafer processing toillustrate in more detail the points of integration of the processingprocess with the detection of profiles of periodic structures andassociated films to illustrate in more detail a part of the process inFIG. 10. As shown in FIG. 11, a semiconductor wafer 352 may be loadedfrom a cassette loader 354 to several stations labeled “prime,” “coat,”“soft bake,” “EBR” etc. Then the wafer 352 is delivered by a stepperinterface 356 to exposure tool 358. The different processes at the fourlocations mentioned above are set forth below:

At the location “Prime”, the wafer undergoes chemical treatment before alayer of photoresist is spun on it, so that the photoresist layer canstick to wafer. At the location “Coat”, a layer of photoresist coatingis spun onto the wafer. At “Soft bake”, the layer of resist is baked toremove chemical solvent from the resist. At “EBR” which stands for“edge-bead removal”, a solvent nozzle or laser is used to remove excessphotoresist from the edge of wafer.

After the wafer has been exposed to radiation by tool 358, the waferthen undergoes four additional processes: “PEB,” “PEB chill,” “Develop,”and “Hard bake.” At “PEB or post exposure bake”, the wafer is baked toreduce standing-wave effect from the exposure tool. Then it is cooled at“PEB chill”. The wafer is then washed with reagent to develop thephotoresist, so that un-exposed (negative) or exposed (positive)photoresist is removed. The wafer then is baked at “Hard bake” tostabilize the photoresist pattern. It will be noted that all of thecomponents of device 350 of FIG. 8 except for the “exposure tool” 358 isknown as the “Track” (also called cluster).

After these latter four processes have been completed, the wafer 352 isthen returned to the cassette loader 354 and this completes theprocessing involving the stepper 350. The detection system 10, 80 or 100may be applied at arrow 362 to measure the parameters of the periodicstructure and associated film(s). Thus, such parameters may be measuredafter “hard bake.”

While the invention has been described above by reference to variousembodiments, it will be understood that changes and modifications may bemade without departing from the scope of the invention, which is to bedefined only by the appended claims and their equivalents. Allreferences referred to herein are incorporated by reference in theirentirety.

1. A method for measuring one or more parameters of a periodicstructure, comprising: directing a polychromatic beam of electromagneticradiation to the structure; collecting radiation from the beam after ithas been modified by the structure; dividing the collected radiationinto two collected rays having different polarization states; detectingthe two rays to provide two outputs; and deriving the one or moreparameters from the two outputs.